1. Field of the Invention
This invention relates generally to the production of regenerated cellulose film and more particularly to a method for treating regenerated cellulose film during the production thereof that enables improved roll formation in the finished product.
2. Prior Art
In the manufacture of regenerated cellulose film, a viscose solution is extruded through a die into an acid coagulating and regenerating bath where the film is cast and then subsequently passed in a serpentine path through additional baths for desulfuring, washing, bleaching, and softening. Thence it is passed through a drying apparatus and wound up into rolls. The details of such process are well known in the prior art and are generally disclosed in U.S. Pat. No. 1,548,864.
One of the problems associated with regenerated cellulose film that has plagued producers from the beginning is the occurrence of thick or raised areas in the film, called gauge variations, running parallel along the length of the film. Individually, such gauge variations are insignificant. However, when the film is wound into rolls, the raised areas pile on top of one another forming gauge bands causing uneven stretching of the film in the vicinity of the bands and concomitant downgrading or loss of value of the finished product. Normally, such gauge variations are caused by slight variations in the die lips which produce large variations in the film as it is extruded. Attempts at correcting the imperfections in the die lips have generally proved unsuccessful or too costly. Other methods of minimizing the problem have also been tried such as scattering the gauge variations using a rotating tubular die as disclosed in U.S. Pat No. 3,280,234. While this method can be used in the production of tubular films, it is not feasible for the extrusion of flat films through the straight parallel lips of an elongated die. Also, U.S. Pat. No. 2,941,255 discloses a method of minimizing the effects of gauge variations in regenerated cellulose film by continuously reciprocating the gel film transversely to the direction of travel during the wet processing stage but prior to application of the softener. This has the effect of scattering some of the imperfections in the film across its width rather than have them pile up over a narrow area. While this technique is effective, it leaves much to be desired where severe gauge variations are encountered, particularly those caused by imperfections in the die lips.